It is well known that hydrocarbons may be produced from subterranean formations through a well that has been drilled into a hydrocarbon bearing formation. Drilling is accomplished by utilizing a drill bit mounted to the end of a drill support member, i.e., a drill string. The drill string is rotated by a top drive, i.e., rotary table, on a surface platform or by a down hole motor mounted towards the lower end of the drill string to facilitate drilling to a desired depth. After reaching the desired depth, the drill string and drill bit are removed, and the wellbore is lined with a string of pipe, i.e., casing. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular space or annulus, i.e., space between two concentric objects, is formed between the string of casing and the wellbore. The casing string is temporarily fixed or “hung” from the surface of the well. A cementing operation is then conducted in order to fill the annular space with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with the drill bit on the drill string. The drill string is removed. A first string of casing is run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. The well is drilled to a second designated depth, and a second string of casing is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. This second casing string is then hung off of the existing casing by slip members and cones to wedgingly fix the second string of casing in the wellbore. The second casing string is then cemented. The process is typically repeated, as necessary with additional casing strings until the well is drilled to the desired depth. As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter in order to fit within the previous casing string. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In many circumstances, it is desirable to alter the direction of the wellbore by drilling one or more additional wellbores (often referred to as “laterals” or “sidetracks”) outward from the primary wellbore in an effort to increase the productivity of the well or to access additional hydrocarbons in adjacent formations. This can be an effective and economical way to substantially increase the profitability of a well and to increase the overall recovery of fluids from a single, primary well site and surface installation. These lateral wells may extend outwardly from the primary wellbore for substantial distances (e.g., 2000 feet or more) or may be relatively short “drainholes” which extend only a few feet (e.g., 100 feet or less) into the formation. However, the ability to drill precisely on target is a significant challenge when drilling laterals. Drill rigs are expensive and several extra days of rig time may substantially reduce the profitability of drilling additional laterals. Efficiently drilling laterals, which directly and precisely exit the primary wellbore at the desired location within the wellbore first, requires cutting an opening or a window through heavy casing or liner.
A conventional technique for drilling laterals may involve the setting of a kickoff plug, or the like, in a primary wellbore. A kickoff plug may have a length ranging from about 50 to about 500 feet, and may comprise a cement composition. The kickoff plug is typically set in the wellbore by lowering a drill string or open-ended tubing string to the desired depth and pumping a cement composition into the wellbore. The cement composition is allowed to cure and form a plug. After the cement plug has formed, a drill string may be used to reinitiate drilling operations. The drill string and drill bit use the plug to drill in a new direction, so as to thereby deflect the drill string and change the direction in which the drilling proceeds. However, the use of kickoff plugs may be problematic due to the fact that the plug prevents access to further production fluids from lower portions of the original wellbore because the cement seals the well at the deviation.
Another conventional method of forming a lateral wellbore employs a whipstock which is inserted into the main wellbore and fixed therein. The whipstock is typically a steel structure that includes a concave, slanted surface along its upper portion arranged to direct drilling tools coming down the wellbore toward one side thereof. In particular, the whipstock forms a guide for gradually directing a cutting device from the main wellbore of the well into and through the wall of the existing wellbore where the new lateral wellbore will be formed or cut. However, similar to the kickoff plug method, whipstocks are typically permanently installed. A conventional permanently installed whipstock may prevent further access to lower formations below the installed whipstock. Furthermore, wells require some amount of work over to remain productive, which may be prevented to some degree by the installation of a permanent whipstock.
When altering the direction of drilling operations, use of small diameter tailpipes may be desired. It is common practice to mill “dual string” window exits through both the tailpipe and the liner in a single milling operation. However, when the clearance between the tailpipe and the liner approach the diameter of the mill, the mill cannot cut through the liner but rather mills down the annulus potentially trapping the mill. Thus, when the inner diameters of the liner and tailpipe have a “high ratio” approaching 2:1, dual string window exits require separate assemblies and at least two oriented milling runs to mill through the tailpipe and liner even though the inner pipe is well anchored by cement. Additionally, cementing operations utilized in current methods of high ratio window exits severely restrict access to lower formations, requiring expensive and risky operations to gain access to these lower formations.
Therefore, a need exits for a process and a system for completing a well to facilitate a sidetrack operation through a liner.